The main objective of this proposal is to use a novel and practical approach to engineer bone marrow cells to display on their surface a modified form of FasL protein with potent apoptotic activity as an immunomodulatory agent and test the capacity of the engineered cells to engraft in allogeneic recipients without complications of graft-versus-host (GVH) or host-versus-graft (HVG) reactions. [unreadable] Allogeneic bone marrow transplantation (BMT) has the potential to cure a series of inherited and acquired hematological disorders and malignancies. BMT can also be used as a cell-based immunomodulatory approach to induce tolerance to foreign and auto-antigens for the prevention and/or treatment of foreign graft rejection and autoimmune disorders. The routine application of BMT as a therapeutic intervention in the clinic, however, is complicated by two different types of immunological reactions; HVG and GVH. HVG reaction is responsible for rejection of allogeneic BM transplants whereas GVH reaction is the major cause of graft versus- host disease (GVHD), which is a life-threatening complication of BMT without effective treatment modalities. T cells specific for alloantigens are the primary culprit in GVH and HVG reactions. Elimination of T cells from the donor BM inoculum or the graft recipient can curtail both types of reactions, lack of engraftment and immunoincompetence are the observed complications. Thus, strategies targeting specific and effective elimination of only the pathogenic T cells may have important implications for routine application of BMT to the clinic for the treatment of a variety of diseases. A novel technique, designated as ProtEx(tm), has recently been developed in our laboratory that allows for rapid (~ 2 hr) and efficient (100% of the targetted cells) display of exogenous proteins of interest on any cell without compromising the function of the cell or the proteins. In this application, ProtEx(tm) will be used to display a modified form of FasL protein with potent apoptotic activity on BM cells and the engineered cells will then be used for transplantation into allogeneic lethally irradiated mice to prevent GVHD and into minimally conditioned hosts to establish mixed chimerism and donor specific tolerance. The use of FasL as an immunomodulatory agent is based on the critical role this molecule plays in immune homeostasis and establishment of self tolerance. We hypothesize that mature T cells in the BM inoculum displaying FasL will respond to the host antigens, upregulate the death receptor Fas, and undergo apoptosis following the engagement of FasL with Fas on the same or a different cell, resulting in the prevention of GVHD. Similarly, recipient T cells reacting to the donor BM cells displaying FasL are expected to undergo apoptosis, leading to engraftment and establishment of mixed chimerism. Rapamycin, an apoptosis-inducing agent, and bisulfan, an alkylating agent that preferentially depletes early hematopoietic stem cells, will be used as adjuvant therapies to i) accentuate FasL-mediated apoptosis, ii) create "space" for donor stem cell engraftment, and iii) establish a clinically applicable protocol. Specific elimination of pathogenic T cells in lethally irradiated or minimally conditioned hosts is expected to results in allogeneic BM engraftment without complications of GVHD and immunoincompetence with strong therapeutic implications. [unreadable] [unreadable]